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Abstract:

An image forming apparatus includes a line head that performs exposure on
a latent image carrier to form a latent image. The line head includes
first and second light-emitting elements arranged in a first direction;
and an optical system that images light emitted from the first and second
light-emitting elements. When a difference between the maximum and
minimum values of a longitudinal aberration of the optical system is G, a
distance in the first direction between centers of geometry of the first
and second light-emitting elements is Pel, and an optical
magnification of the optical system is β, a relation of
G>|β|Pel is satisfied.

Claims:

1. A line head comprising:first and second light-emitting elements that
are arranged in a first direction; andan optical system that images light
emitted from the first and second light-emitting elements, whereinwhen a
difference between the maximum and minimum values of a longitudinal
aberration of the optical system is defined as G, a distance in the first
direction between centers of geometry of the first and second
light-emitting elements is defined as Pel, and an optical
magnification of the optical system is defined as β, the following
relation is satisfied:G>|β|Pel.

2. The line head according to claim 1, wherein:the optical system has a
lens surface that includes first and second regions with surface shapes
that are defined by different definition formulas;the second region
surrounds the first region in a ring shape; andwhen an imaging point on
an optical axis is used as a reference, and a light traveling direction
of the optical axis is defined as a positive direction, the optical
system is configured such that when, among the light emitted from the
first light-emitting element, the minimum value of a longitudinal
aberration of light passing through the first region is defined as
Δ1, and the maximum value of light passing through the second
region is defined as Δ2, the following relation is
satisfied:Δ.sub.2-.DELTA.1=G.

3. The line head according to claim 2, wherein:three or more
light-emitting elements including the first and second light-emitting
elements are arranged in the first direction; andthe first and second
light-emitting elements are adjacent to each other in the first
direction.

4. The line head according to claim 2, wherein:the lens surface includes a
third region that is defined by a definition formula different from that
of the first region and includes an intersection with the optical axis;
andthe first region surrounds the third region in a ring shape.

5. The line head according to claim 2, wherein the minimum value
Δ1 of the longitudinal aberration of light passing through the
first region has a negative sign, and the maximum value of the
longitudinal aberration Δ2 of light passing through the second
region has a positive sign.

6. The line head according to claim 2, wherein an aperture diaphragm is
provided close to a front-side focal point of the optical system.

7. The line head according to claim 6, wherein the first and second
regions are included in a lens surface that is located closest to the
aperture diaphragm among the lens surfaces of the optical system.

8. An image forming apparatus comprising:a latent image carrier on which a
latent image is formed; anda line head that performs exposure on the
latent image carrier so as to form the latent image, whereinthe line head
comprises:first and second light-emitting elements that are arranged in a
first direction; andan optical system that images light emitted from the
first and second light-emitting elements, whereinwhen a difference
between the maximum and minimum values of a longitudinal aberration of
the optical system is defined as G, a distance in the first direction
between centers of geometry of the first and second light-emitting
elements is defined as Pel, and an optical magnification of the
optical system is defined as β, the following relation is
satisfied:G>|β|Pel.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the benefit of priority under 35 USC 119 of
Japanese application no. 2009-032045, filed on Feb. 13, 2009, which is
incorporated herein by reference.

BACKGROUND

[0002]1. Technical Field

[0003]The present invention relates to a line head and an image forming
apparatus.

[0004]2. Related Art

[0005]Electrophotographic image forming apparatuses such as copying
machines or printers are provided with an exposure unit that performs an
exposure process on an outer surface of a rotating photoconductor so as
to form an electrostatic latent image thereon. As the exposure unit, a
line head having a structure in which a plurality of light-emitting
elements is arranged in the direction of the rotation axis of the
photoconductor is known (for example, see JP-A-2-4546)

[0006]As the line head, for example, JP-A-2-4546 describes an optical
information writer in which a plurality of LED array chips with a
plurality of LEDs (light-emitting elements) is arranged in one direction.

[0007]In the optical information writer, the plurality of LEDs of each of
the LED array chips is arranged in the direction of the rotation axis of
the photoconductor. Convex lens elements (optical systems) are provided
so as to correspond to the respective LED array chips. The convex lens
elements image the rays of light from the respective LEDs of each of the
LED array chip.

[0008]In the line head described in JP-A-2-4546, due to the image-surface
curvature of the convex lens element, the imaging capability of the
convex lens element decreases as it becomes distant from the optical
axis. On the surface of the photoconductor, a spot size of light from an
LED that is located close to the optical axis of the convex lens element
is different from a spot size of light from an LED that is located
distant from the optical axis of the convex lens element. That is, the
size of the latent image formed on the surface of the photoconductor
becomes different from a pixel, which is formed by the light from the LED
located close to the optical axis of the convex lens element, to a pixel,
which is formed by the light from the LED located distant from the
optical axis of the convex lens element. The images obtained by
developing the latent image may have uneven concentration between the two
pixels, and thus concentration unevenness occurs.

[0009]Furthermore, since the positional relationship between the image
surface of the convex lens element and the light irradiation surface (the
surface of the photoconductor) is offset or varied due to errors in
mounting the line head onto the body of the image forming apparatus,
eccentricity of the photoconductor, or the like, the spot size will vary.
In this respect, the obtained images will have concentration unevenness.

SUMMARY

[0010]An advantage of some aspects of the invention is that it provides a
line head that performs a high-accuracy exposure process and an image
forming apparatus that obtains a high-quality image.

[0011]The above-described advantage is achieved by the following aspects
and embodiments of the invention.

[0012]According to an aspect of the invention, a line head includes first
and second light-emitting elements that are arranged in a first
direction; and an optical system that images light emitted from the first
and second light-emitting elements. When a difference between the maximum
and minimum values of a longitudinal aberration of the optical system is
defined as G, a distance in the first direction between centers of
geometry of the first and second light-emitting elements is defined as
Pel, and an optical magnification of the optical system is defined
as β, a relation of G>|β|Pel is satisfied.

[0013]In one embodiment, the optical system has a lens surface that
includes first and second regions with surface shapes that are defined by
different definition formulas. The second region surrounds the first
region in a ring shape. When an imaging point on an optical axis is used
as a reference, and a light traveling direction of the optical axis is
defined as a positive direction, the optical system is configured such
that when, among the light emitted from the first light-emitting element,
the minimum value of a longitudinal aberration of light passing through
the first region is defined as Δ1, and the maximum value of
light passing through the second region is defined as Δ2, a
relation of Δ2-Δ1=G is satisfied.

[0014]In another embodiment, three or more light-emitting elements
including the first and second light-emitting elements are arranged in
the first direction, and the first and second light-emitting elements are
adjacent to each other in the first direction.

[0015]In another embodiment, the lens surface includes a third region that
is defined by a definition formula different from that of the first
region and that includes an intersection with the optical axis. The first
region surrounds the third region in a ring shape.

[0016]In another embodiment, the minimum value Δ1 of the
longitudinal aberration of light passing through the first region has a
negative sign, and the maximum value of the longitudinal aberration
Δ2 of light passing through the second region has a positive
sign.

[0017]In another embodiment, an aperture diaphragm is provided close to a
front-side focal point of the optical system.

[0018]In another embodiment, the first and second regions are included in
a lens surface that is located closest to the aperture diaphragm among
the lens surfaces of the optical system.

[0019]According to another aspect of the invention, an image forming
apparatus includes a latent image carrier on which a latent image is
formed; and a line head that performs exposure on the latent image
carrier so as to form the latent image. The line head includes first and
second light-emitting elements that are arranged in a first direction;
and an optical system that images light emitted from the first and second
light-emitting elements. When a difference between the maximum and
minimum values of a longitudinal aberration of the optical system is
defined as G, a distance in the first direction between centers of
geometry of the first and second light-emitting elements is defined as
Pel, and an optical magnification of the optical system is defined
as β, a relation of G>|β|Pel is satisfied.

[0020]According to a line head of the invention having the above-described
configuration, when light emitted from the light-emitting element is
imaged by the optical system, the spot size of the light is made
substantially constant over a relatively wide range in the optical axis
direction in the vicinity of the image surface (that is, the focal depth
is increased). Therefore, even when the positional relationship in the
optical axis direction between the image surface and the light
irradiation surface, is changed or offset, variation of the spot size on
the light irradiation surface is prevented. As a result, a high-accuracy
exposure process is realized.

[0021]Moreover, according to the image forming apparatus of the invention,
by realizing the above-described high-accuracy exposure process, a
high-quality image is obtained in which concentration unevenness is
suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]The invention will be described with reference to the accompanying
drawings, wherein like numbers reference like elements.

[0023]FIG. 1 is a schematic view of an image forming apparatus according
to an embodiment of the invention.

[0024]FIG. 2 is a partially sectional perspective view of a line head
included in the image forming apparatus of FIG. 1.

[0025]FIG. 3 is a cross-sectional view taken along line III-III of FIG. 2.

[0026]FIG. 4 is a plan view of the line head of FIG. 2, illustrating the
positional relationship between lenses and light-emitting elements.

[0027]FIG. 5. is a cross-sectional view, taken along the main-scanning
direction, of an optical system included in the line head of FIG. 2.

[0028]FIG. 6 is a cross-sectional view, taken along the main-scanning
direction, for describing an optical magnification of the optical system
included in the line head of FIG. 2.

[0029]FIG. 7 is a plan view illustrating another example of the
arrangement of light-emitting elements.

[0030]FIGS. 8A and 8B are views illustrating a light-emitting element-side
lens included in the optical system of FIG. 5.

[0031]FIG. 9 is a view for describing the operation of the lens of FIGS.
8A and 8B.

[0032]FIG. 10 is a view for describing the operation of the optical system
of FIG. 5.

[0033]FIGS. 11A and 11B are views illustrating another example of the
light-emitting element-side lens included in the optical system of FIG.
5.

[0034]FIG. 12 is a view illustrating an optical system included in a line
head according to an Example of the invention.

[0035]FIG. 13 is a graph illustrating the longitudinal aberration of an
optical system included in a line head according to the Example of the
invention.

[0036]FIGS. 14A and 14E are graphs illustrating the spot sizes in the
vicinity of a photoconductor surface (an image surface), respectively, of
the optical system of the line head of the Example of the invention and
an optical system of a line head of a Comparative Example.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0037]A line head and image forming apparatus according to embodiments of
the invention are now described in detail with reference to the
accompanying drawings.

[0038]FIG. 1 is a schematic view of an image forming apparatus according
to an embodiment of the invention. FIG. 2 is a partially sectional
perspective view of a line head included in the image forming apparatus
of FIG. 1. FIG. 3 is a cross-sectional view taken along line of FIG. 2.
FIG. 4 is a plan view of the line head of FIG. 2, illustrating the
positional relationship between lenses and light-emitting elements. FIG.
5 is a cross-sectional view, taken along the first direction, of an
optical system included in the line head of FIG. 2. FIG. 6 is a
cross-sectional view, taken along the main-scanning direction, for
describing an optical magnification of the optical system included in the
line head of FIG. 2. FIG. 7 is a plan view illustrating another example
of the arrangement of light-emitting elements. FIGS. 8A and 8B are views
illustrating a light-emitting element-side lens included in the optical
system of FIG. 5. FIG. 9 is a view for describing the operation of the
lens of FIGS. 8A and 8B. FIG. 10 is a view for describing the operation
of the optical system of FIG. 5. FIGS. 11A and 11B are views illustrating
another example of the light-emitting element-side lens included in the
optical system of FIG. 5. In the following description, an upper side in
FIGS. 1-3 and FIG. 5 is referred to as "upper" or "upward" and a lower
side in the drawings is referred to as "lower" or "downward" for
convenience of explanation.

Image Forming Apparatus

[0039]Image forming apparatus 1 of FIG. 1 is an electrophotographic
printer that records an image on a recording medium P by a series of
image forming processes including electrical charging, exposure,
developing, transferring and fixing processes. In this embodiment, the
image forming apparatus 1 is a tandem type color printer.

[0040]As illustrated in FIG. 1, the image forming apparatus 1 includes: an
image forming unit 10 for the electrical charging process, the exposure
process, and the developing process; a transfer unit 20 for the
transferring process; a fixing unit 30 for the fixing process; a
transport mechanism 40 for transporting the recording medium P, such as
paper; and a paper feed unit 50 that supplies the recording medium P to
the transport mechanism 40.

[0041]The image forming unit 10 has four image forming stations: an image
forming station 10Y that forms a yellow toner image, an image forming
station 10M that forms a magenta toner image, an image forming station
10C that forms a cyan toner image, and an image forming station 10K that
forms a black toner image.

[0042]Each of the image forming stations 10Y, 100, 10M, and 10K has a
photosensitive drum (photoconductor) 11 that is an electrostatic latent
image carrier that carries an electrostatic latent image thereon. A
charging unit 12, a line head (exposure unit) 13, a developing unit 14,
and a cleaning unit 15 are provided around the periphery (outer
peripheral side) of the photosensitive drum 11 along a rotating direction
thereof. The image forming stations 10Y, 10C, 10M, and 10K have
substantially the same configurations except that they use toner of
different colors.

[0043]The photosensitive drum 11 has an overall cylindrical shape and is
rotatable around an axial line thereof along the direction indicated by
the arrow in FIG. 1. A photosensitive layer is formed in the vicinity of
the outer peripheral surface (cylindrical surface) of the photosensitive
drum 11. The outer peripheral surface of the photosensitive drum 11 forms
a light receiving surface 111 that receives light L (emitted light) from
the line head 13.

[0045]The line head 13 receives image information from a host computer
such as a personal computer and irradiates the light L towards the light
receiving surface 111 of the photosensitive drum 11 in response to the
image information. When the light L is irradiated to the uniformly
charged light receiving surface 111 of the photosensitive drum 11, a
latent image (electrostatic latent image) corresponding to an irradiation
pattern of the light L is formed on the light receiving surface 111. The
configuration of the line head 13 is described in detail later.

[0046]The developing unit 14 has a reservoir storing toner therein and
supplies toner from the reservoir to the light receiving surface 111 of
the photosensitive drum 11 that carries the electrostatic latent image
and applies toner thereon. As a result, the latent image on the
photosensitive drum 11 is visualized (developed) as a toner image.

[0047]The cleaning unit 15 has a cleaning blade 151, which is made of
rubber and makes abutting contact with the light receiving surface 111 of
the photosensitive drum 11, and is configured to remove toner, which
remains on the photosensitive drum 11 after a primary transfer that is
described later, by scraping the remaining toner with the cleaning blade
151.

[0049]In each of the image forming stations 10Y, 10C, 10M, and 10K,
electrical charging of the light receiving surface 111 of the
photosensitive drum 11 performed by the charging unit 12, exposure of the
light receiving surface 111 performed by the line head 13, supply of
toner to the light receiving surface 111 performed by the developing unit
14, primary transfer to an intermediate transfer belt 21, caused by
pressure between the intermediate transfer belt 21 and a primary transfer
roller 22, which will be described later, and cleaning of the light
receiving surface 111 performed by the cleaning unit 15 are sequentially
performed while the photosensitive drum 11 rotates once.

[0050]The transfer unit 20 has the intermediate transfer belt 21 having an
endless belt shape. The intermediate transfer belt 21 is stretched over
the plurality (four in the configuration illustrated in FIG. 1) of
primary transfer rollers 22, a driving roller 23, and a driven roller 24.
The intermediate transfer belt 21 is driven to rotate in the direction
indicated by the arrow illustrated in FIG. 1 and at approximately the
same speed as a circumferential speed of the photosensitive drum 11 by
rotation of the driving roller 23.

[0051]Each primary transfer roller 22 is provided opposite the
corresponding photosensitive drum 11 with the intermediate transfer belt
21 interposed therebetween and is configured to transfer (primary
transfer) a monochrome toner image on the photosensitive drum 11 to the
intermediate transfer belt 21. At the time of primary transfer, a primary
transfer voltage (primary transfer bias), which has an opposite polarity
to that of electrically charged toner is applied to the primary transfer
roller 22.

[0052]A toner image corresponding to at least one of the colors yellow,
magenta, cyan, and black is carried on the intermediate transfer belt 21.
For example, when a full color image is formed, toner images
corresponding to the four colors yellow, magenta, cyan, and black are
sequentially transferred onto the intermediate transfer belt 21 so as to
overlap one another so that a full color toner image is formed as an
intermediate transfer image.

[0053]In addition, the transfer unit 20 has a secondary transfer roller
25, which is provided opposite the driving roller 23 with the
intermediate transfer belt 21 interposed therebetween, and a cleaning
unit 26, which is provided opposite the driven roller 24 with the
intermediate transfer belt 21 interposed therebetween.

[0054]The secondary transfer roller 25 is configured to transfer
(secondary transfer) a monochrome or full-color toner image (intermediate
transfer image), which is formed on the intermediate transfer belt 21, to
the recording medium P such as paper, a film, or cloth, which is supplied
from the paper feed unit 50. At the time of secondary transfer, the
secondary transfer roller 25 is pressed against the intermediate transfer
belt 21, and a secondary transfer voltage (secondary transfer bias) is
applied to the secondary transfer roller 25. The driving roller 23 also
functions as a backup roller of the secondary transfer roller 25 at the
time of such secondary transfer.

[0055]The cleaning unit 26 has a cleaning blade 261, which is made of
rubber and makes abutting contact with a surface of the intermediate
transfer belt 21, and is configured to remove toner, which remains on the
intermediate transfer belt 21 after the secondary transfer, by scraping
the remaining toner with the cleaning blade 261.

[0056]The fixing unit 30 has a fixing roller 301 and a pressure roller 302
pressed against the fixing roller 301 and is configured such that the
recording medium P passes between the fixing roller 301 and the pressure
roller 302. In addition, the fixing roller 301 is provided with a heater
at an inside thereof to heat an outer peripheral surface of the fixing
roller 301 so that the recording medium P passing between the fixing
roller 301 and the pressure roller 302 can be heated and pressed. By the
fixing unit 30 having such a configuration, the recording medium P having
a secondary-transferred toner image thereon is heated and pressed, such
that the toner image is heat-fixed on the recording medium P as a
permanent image.

[0057]The transport mechanism 40 has a resist roller pair 41, which
transports the recording medium P to a secondary transfer position while
calculates the timing of paper feeding to the secondary transfer position
between the secondary transfer roller 25 and the intermediate transfer
belt 21 described above, and transport roller pairs 42, 43, and 44 which
pinch and transport only the recording medium P, on which the fixing
process in the fixing unit 30 has been completed.

[0058]When an image is formed on only one surface of the recording medium
P, the transport mechanism 40 pinches and transports the recording medium
P, in which one surface thereof has been subjected to the fixing process
by the fixing unit 30, using the transport roller pair 42 and discharges
the recording medium P to the outside of the image forming apparatus 1.
When images are formed on both surfaces of the recording medium P, the
recording medium P in which one surface thereof has been subjected to the
fixing process by the fixing unit 30 is first pinched by the transport
roller pair 42. Then, the transport roller pair 42 is reversely driven
and the transport roller pairs 43 and 44 are driven so as to reverse the
recording medium P upside down and transport the recording medium P back
to the resist roller pair 41. Then, another image is formed on the other
surface of the recording medium P by the same operation as described
above.

[0059]The paper feed unit 50 is provided with a paper feed cassette 51,
which stores therein the recording medium P that has not been used, and a
pickup roller 52 that feeds the recording medium P from the paper feed
cassette 51 toward the resist roller pair 41 one at a time.

Line Head

[0060]The line head 13 is now described in detail. In the following
description, the longitudinal direction (first direction) of a long lens
array 6 is referred to as a "main-scanning direction" and the width
direction (second direction) of the lens array 6 is referred to as a
"sub-scanning direction" for convenience of explanation.

[0061]As illustrated in FIG. 3, the line head 13 is arranged below the
photosensitive drum 11 so as to oppose the light receiving surface 111 of
the photosensitive drum 11. The line head 13 includes a lens array (first
lens array) 6', a spacer 84, the lens array (second lens array) 6, a
light shielding member (first light shielding member) 82, a diaphragm
member (aperture diaphragm) 83, a light shielding member (second light
shielding member) 81, and a light-emitting element array 7, which are
sequentially arranged in that order from the side of the photosensitive
drum 11 and are accommodated in a casing 9.

[0062]In the line head 13, the light L emitted from the light-emitting
element array 7 is collimated by the diaphragm member 83 and sequentially
passes through the lens array 6' and the lens array 6 to be irradiated
onto the light receiving surface 111 of the photosensitive drum 11.

[0063]As illustrated in FIG. 2, the lens arrays 6 and 6' are formed of a
planar member having a long appearance.

[0064]As illustrated in FIG. 3, a plurality of lens surfaces (convex
surfaces) 62 is formed on a lower surface (incidence surface) of the lens
array 6 on which the light L is incident. On the other hand, an upper
surface (emission surface) of the lens array 6 from which the light L is
emitted is configured as a flat surface.

[0065]That is, the lens array 6 includes a plurality of plano-convex
lenses 64, each of the lenses having a convex surface on a surface on
which the light L is incident and a flat surface on a surface from which
the light L is emitted. A portion of the lens array 6 excluding the
respective lenses 64 constitutes a support portion 65 that supports each
of the lenses 64.

[0066]Similarly, on a lower surface (incidence surface) of the lens array
6' on which the light L is incident, a plurality of lens surfaces (convex
surfaces) 62' is formed so as to correspond to the plurality of lens
surfaces 62 described above. On the other hand, an upper surface
(emission surface) of the lens array 6' from which the light L is emitted
is configured as a flat surface.

[0067]That is, the lens array 6' includes a plurality of piano-convex
lenses 64', each of the lenses having a convex surface on a surface on
which the light L is incident and a flat surface on a surface from which
the light L is emitted. A portion of the lens array 6' excluding the
respective lenses 64' constitutes a support portion 65' that supports
each of the lenses 64'.

[0068]A plurality of lens pairs 64 and 64' constitutes an optical system
60 that images light emitted from corresponding light-emitting elements
74 of a light-emitting element group 71 (see FIGS. 5 and 6). The optical
system 60 (particularly, the shapes of the lens surfaces of the lenses 64
and 64') is described in detail later.

[0069]The arrangement of the lenses 64 is now described. Since the lenses
64' have the same arrangement (in plan view) as the lenses 64, a
description thereof is omitted.

[0070]As illustrated in FIG. 4, the lenses 64 are arranged in plural
columns in the main-scanning direction (first direction), and are
arranged in plural rows in the sub-scanning direction (second direction)
that is orthogonal to the main-scanning direction and the optical axis
direction of the lenses 64.

[0071]More specifically, the plurality of lenses 64 are arranged in a
matrix of three rows by n columns (n is an integer of two or more). In
the following description, among the three lenses 64 belonging, to one
column (lens array), the lens 64 positioned in the middle is referred to
as a "lens 64b", the lens 64 positioned at a left side in FIG. 3 (upper
side in FIG. 4) is referred to as a "lens 64a", and the lens 64
positioned at a right side in FIG. 3 (lower side in FIG. 4) is referred
to as a "lens 64c". In the. lenses 64' that are paired with the lenses
64, the lens 64' corresponding to the lens 64a is referred to as a "lens
64a'", the lens 64' corresponding to the lens 64b is referred to as a
"lens 64b'", and the lens 64' corresponding to the lens 64c is referred
to as a "lens 64c'".

[0072]In this embodiment, the line head 13 is mounted on the image forming
apparatus 1 so that, among the plural lenses 64 (64a to 64c) belonging to
one column, the lens 64b positioned closest to the center in the
sub-scanning direction is arranged at the position closed to the light
receiving surface 111 of the photosensitive drum 11. By doing so, the
optical characteristics of the plurality of lenses 64 can be configured
easily.

[0073]As illustrated in FIGS. 2 and 4, in each lens column, the lenses 64a
to 64c are sequentially arranged so as to be offset by an equal distance
in the main-scanning direction (right direction in FIG. 4). That is, in
each lens column, a line that connects the centers of the lenses 64a to
64c to one another is inclined at a predetermined angle with respect to
the main-scanning direction and the sub-scanning direction.

[0074]When seen from the cross section of FIG. 3, the three lenses 64
belonging to one lens column, namely the lenses 64a and 64c, are arranged
such that the optical axes 601 of the lenses 64a and 64c are symmetrical
with respect to the optical axis 601 of the lens 64b. Moreover, the
optical axes 601 of the lenses 64a to 64c are arranged in parallel to
each other.

[0075]Although the constituent materials of the lens arrays 6 and 6' are
not particularly limited as long as they exhibit the optical
characteristics described above, the lens arrays 6 and 6' are preferably
formed of a resin material and/or a glass material, for example.

[0076]As the resin material, various kinds of resin materials can be used.
Examples thereof include liquid crystal polymers such as polyamides,
thermoplastic polyimides and polyamideimide aromatic polyesters;
polyolefins such as polyphenylene oxide, polyphenylene sulfide and
polyethylene; polyesters such as modified polyolefins, polycarbonate,
acrylic (methacrylic) resins, polymethyl methacrylate, polyethylene
terephthalate and polybutylene terephthalate; thermoplastic resins such
as polyethers, polyether ether ketones, polyetherimide and polyacetal;
thermosetting resins such as epoxy resins, phenolic resins, urea resins,
melamine resins, unsaturated polyester resins and polyimide resins;
photocurable resins; and the like. These can be used individually or in
combination of two or more species.

[0077]Among these resin materials, resin materials such as thermosetting
resins and photocurable resins are preferred because such materials have
a relative low thermal expansion coefficient and are rarely thermally
expanded (deformed), modified or deteriorated, in addition to the
advantages of a relative high refractive index.

[0078]As the glass material, various kinds of glass materials, such as
soda glass, crystalline glass, quartz glass, lead glass, potassium glass,
borosilicate glass, alkali-free glass, and the like may be used. When a
support portion 72 of the light-emitting element array 7 is formed of a
glass material, the lens arrays 6 and 6' is preferably formed of a glass
material having approximately the same linear expansion rate as the above
glass material. By doing so, the positional misalignment of the
respective lenses relative to the light-emitting elements due to
temperature variation can be prevented.

[0079]When the lens array 6 is formed by using a combination of the
described resin and glass materials, a glass substrate formed of a glass
material may be used as the support portion 65, for example, as is
described later. In this case, a resin layer formed of a resin material
may be formed on one surface of the glass substrate, and the lens surface
62 may be formed on the other surface of the glass substrate opposite the
resin layer, thus forming the lens 64 (see FIGS. 5 and 6). The lens array
6 may be obtained, for example, by forming a plurality of convex
portions, which is formed of a resin material and protrudes in a convex
surface shape, on one surface of a flat plate-like member (substrate)
that is formed of a glass material.

[0080]As illustrated in FIGS. 2 and 3, a spacer 84 is provided between the
lens arrays 6 and 6'. The lens arrays 6 and 6' are bonded together via
the spacer 84.

[0081]The spacer 84 has a function of regulating a gap length that is a
distance between the lens arrays 6 and 6'.

[0082]By adjusting the thickness of the spacer 84, the separation distance
between the lenses 64 and 64' can be adjusted to a desired value.

[0083]The spacer 84 has a frame shape that corresponds to the outer
peripheral portions of the lens arrays 6 and 6' and is bonded to these
peripheral portions. The spacer 84 is not limited to a frame-shaped
member as long as it has the above-described function. The spacer 84 may
be configured as a pair of members which correspond to one of the
opposing sides of the outer peripheral portions of the lens arrays 6 and
6'. Alternatively, the spacer 84 may be configured as a planar member
having through-holes formed therein so as to correspond to optical paths,
similar to light shielding members 81 and 83 which are described later.

[0084]Although the constituent materials of the spacer 84 are not
particularly limited as long as they exhibit the above-described
function, a resin material, a metallic material, a glass material, a
ceramics material, and the like can be used, for example.

[0085]As illustrated in FIG. 3, at a side of the lens array 6 on which the
light L is incident, the light-emitting element array 7 is provided with
the light shielding member 82, a diaphragm member 83, and the light
shielding member 81 interposed therebetween. The light-emitting element
array 7 has a plurality of groups of light-emitting elements
(light-emitting element groups) 71 and a supporting plate (head
substrate) 72.

[0086]The supporting plate 72 supports each of the light-emitting element
groups 71 and is formed of a planar member having a long appearance. The
supporting plate 72 is arranged in parallel to the lens array 6.

[0087]The length of the supporting plate 72 in the main-scanning direction
is longer than that of the lens array 6 in the main-scanning direction.
The length of the supporting plate 72 in the sub-scanning direction is
also set to be longer than that of the lens array 6 in the sub-scanning
direction.

[0088]Although the constituent materials of the supporting plate 72 are
not particularly limited, when the light-emitting element groups 71 are
provided on the bottom surface side of the support portion 72 (that is,
bottom emission-type light-emitting elements are used as the
light-emitting elements 74), the supporting plate 72 is preferably formed
of transparent materials such as various kinds of glass materials or
plastics. When top emission-type light-emitting elements are used as the
light-emitting elements 74, the constituent materials of the support
portion 72 are not limited to transparent materials, and various kinds of
metallic materials, such as aluminum or stainless steel, various kinds of
glass materials, various kinds of plastics, and the like may be used
individually or in combination thereof. When the supporting plate 72 is
formed of various kinds of metallic materials or glass materials, heat
generated by emission of the light-emitting elements 74 is efficiently
dissipated through the supporting plate 72. When the supporting plate 72
is formed of various kinds of plastics, the weight of the supporting
plate 72 is reduced.

[0089]A box-shaped accommodation portion 73 which is open to the support
portion 72 is provided on the bottom surface side of the supporting plate
72. The plurality of light-emitting element groups 71, wiring lines
electrically connected to the light-emitting element groups 71 (the
respective light-emitting elements 74), or circuits used for driving the
respective light-emitting elements 74 are accommodated in the
accommodation portion 73.

[0090]The plurality of light-emitting element groups 71 are separated from
each other and arranged in a matrix of three rows by n columns (n is an
integer of two or more) so as to correspond to the plurality of lenses 64
described above (for example, see FIG. 4). Each of the light-emitting
element groups 71 includes a plurality (eight in this embodiment) of
light-emitting elements 74.

[0091]The eight light-emitting elements 74 that constitute each of the
light-emitting element groups 71 are arranged along a lower surface 721
of the supporting plate 72 of FIG. 3. The light L emitted from each of
the eight light-emitting elements 74 is focused (imaged) on the light
receiving surface 111 of the photosensitive drum 11 through the
corresponding lens 64.

[0092]As illustrated in FIG. 4, the eight light-emitting elements 74 are
separated from each other and are arranged in four columns in the
main-scanning direction and in two rows in the sub-scanning direction.
Thus, the eight light-emitting elements 74 are arranged in a matrix of
two rows by four columns. The two adjacent light-emitting elements 74
belonging to one column (column of light-emitting elements) are offset
from each other in the main-scanning direction.

[0093]In the eight light-emitting elements 74 that form a matrix of two
rows by four columns, two light-emitting elements 74 that are adjacent to
each other in the main-scanning direction are supplemented by one
light-emitting element 74 in a next row.

[0094]There is a limitation in arranging the eight light-emitting elements
74 as closely as possible in one row. However, the arrangement density of
the light-emitting elements 74 can be further increased by arranging the
eight light-emitting elements 74 so as to be offset from each other as
described above. In this way, the recording density of the recording
medium P when an image is recorded on the recording medium P is increased
further. As a result, a recording medium P carrying thereon an image that
has high resolution and multiple gray-scale levels and is clear is
obtained.

[0095]Although the eight light-emitting elements 74 belonging to one
light-emitting element group 71 are arranged in a matrix of two rows by
four columns in this embodiment, the arrangement shape is not limited
thereto. For example, the eight light-emitting elements 74 may be
arranged in a matrix of four rows by two columns.

[0096]As described above, the plurality of light-emitting element groups
71 are arranged in a matrix of three rows by n columns so as to be
separated from each other. As illustrated in FIG. 4, the three
light-emitting element groups 71 belonging to one column (column of
light-emitting element groups) are offset from each other by an equal
distance in the main-scanning direction (right direction in FIG. 4).

[0097]Thus, in the light-emitting element groups 71 that form a matrix of
three rows by n columns, the gaps between adjacent light-emitting element
groups 71 are sequentially supplemented by the light-emitting element
group 71 of a next row and the light-emitting element group 71 of a
subsequent row.

[0098]There is a limitation in arranging the plurality of light-emitting
element groups 71 as closely as possible in one row. However, the
arrangement density of the light-emitting element groups 71 can be
further increased by arranging the plurality of light-emitting element
groups 71 so as to be offset from each other as described above. In this
way, by the synergetic effect with the fact that the eight light-emitting
elements 74 within one light-emitting element group 71 are offset from
each other, the recording density of the recording medium P when an image
is recorded on the recording medium P is increased further. As a result,
a recording medium P carrying thereon an image that has higher
resolution, multiple gray-scale levels, high color reproducibility and is
clearer is obtained.

[0099]The light-emitting elements 74 may be bottom emission-type organic
electroluminescence (EL) element. However, the light-emitting elements 74
are not limited to bottom emission-type elements and may be top
emission-type elements. In this case, the support portion 72 is not
required to have optically transparent properties as described above.

[0100]When the light-emitting elements 74 are organic EL elements, the
gaps (pitches) between the light-emitting elements 74 can be set to be
relatively small. In this way, the recording density of the recording
medium P when an image is recorded on the recording medium P is made
relatively high. In addition, the light-emitting elements 74 can be
formed with highly accurate sizes and at highly accurate positions by
using various film-forming methods. As a result, a recording medium P
carrying thereon a clearer image is obtained.

[0101]In this embodiment, all of the light-emitting elements 74 are
configured to emit red light. Here,
(4-dicyanomethylene)-2-methyl-6-paradimethylaminostyryl)-4H-pyrane (DCM),
Nile Red and the like are examples of constituent materials of a
light-emitting layer that emits red light. However, the light-emitting
elements 74 are not limited to those that emit red light, and may be
configured to emit monochromatic light of another color or white light.
Thus, in the organic EL element, the light L emitted from the
light-emitting layer can be appropriately set to monochromatic light of
an arbitrary color in accordance with the constituent materials of the
light-emitting layer.

[0102]Since the spectral sensitivity characteristic of the photosensitive
drum used in the electrophotographic process is generally set to have a
peak in a wavelength range of a red wavelength, which is the emission
wavelength of a semiconductor laser, to a near-red wavelength, materials
capable of emitting red light as described above are preferably used.

[0103]As illustrated in FIG. 3, the light shielding member 82, the
diaphragm member 83, and the light shielding member 81 are provided
between the lens array 6 and the light-emitting element array 7.

[0104]The light shielding members 81 and 82 prevent crosstalk of the light
L between the adjacent light-emitting element groups 71.

[0105]A plurality of through-holes (openings) 811 is formed in the light
shielding member 81 so as to pass through the light shielding member 81
in the up and down direction (thickness direction) of FIG. 3.
Through-holes 811 are arranged at positions corresponding to the
respective lenses 64.

[0106]Similarly, a plurality of through-holes 821 is formed in the light
shielding member 82 so as to pass through the light shielding member 82
in the up and down direction (thickness direction) of FIG. 3.
Through-holes 821 are arranged at positions corresponding to the
respective lenses 64.

[0107]Each of the through-holes 811 and 821 forms an optical path that
extends from the light-emitting element group 71 to the corresponding
lens 64. In addition, each of the through-holes 811 and 821 has a
circular shape in plan view thereof and includes therein the eight
light-emitting elements 74 of the light-emitting element group 71
corresponding to each of the through-holes 811 and 821.

[0108]Although the through-holes 811 and 821 have a cylindrical shape in
the configuration of FIG. 3, the invention is not limited thereto. For
example, the through-holes 811 and 821 may have a circular truncated cone
shape that expands upward.

[0109]The diaphragm member 83 is provided between the light shielding
members 81 and 82.

[0110]The diaphragm member 83 is an aperture diaphragm that restricts the
amount of light L incident on the lens 64 from the light-emitting element
group 71 to a predetermined amount.

[0111]In particular, the diaphragm member 83 is disposed in the vicinity
of a front-side focal plane of the optical system 60. Therefore, the
optical system 60 becomes telecentric on the image side. Even when there
is an error in the distance between the light receiving surface 111 and
the line head 13 due to mounting errors or the like, deviation of an
imaging position in each of the light-emitting elements 74a, 74d, 74b,
and 74c (first and second light-emitting elements) can be prevented.

[0112]The diaphragm member 83 has a planar or layered shape, and a
plurality of through-hole (openings) 831 is formed in the diaphragm
member 83 so as to pass through the diaphragm member 83 in the up and
down direction (thickness dimension) of FIG. 3. Through-holes 831 are
arranged at positions corresponding to the lenses 64 (namely, the
above-described through-holes 811 and 821).

[0113]In addition, each of the through-holes 831 of the diaphragm member
83 has a circular shape in plan view thereof and has a diameter smaller
than that of the through-holes 811 of the light shielding member 81
described above.

[0114]The diaphragm member 83 is preferably configured to set a distance
to the lens 64 to be relatively small. By doing so, light emitted from
light-emitting elements 74 that are located at different distances from
the optical axis 601 (that is, even when the light-emitting elements 74
are located at different angles of view) can be made incident to
approximately the same region of the lens 64.

[0115]The light shielding members 81 and 82 and the diaphragm member 83
also have a function of regulating the distance, positional relationship,
and attitude between the lens array 6 and the support portion 72 with
high accuracy.

[0116]The distance between the lens surface 62 of each lens 64 and the
corresponding light-emitting element group 71 is an important condition
(element) that determines the position in the up and down direction of
FIG. 3 of the imaging position of the optical system 60. Therefore, as
described above, when the light shielding members 81 and 82 and the
diaphragm member 83 function as the spacer that regulates the gap length
that is the distance between the lens array 6 and the light-emitting
element array 7, an image forming apparatus 1 that is highly precise and
reliable is obtained.

[0117]Moreover, the light shielding member 81 and 82 and the diaphragm
member 83 preferably have at least an inner peripheral surface thereof
that has a dark color such as black, brown, or dark blue.

[0118]Although the constituent materials of the light shielding members 81
and 82 and the diaphragm member 83 are not particularly limited as long
as they are not optically transparent, various kinds of coloring agents,
metallic materials such as chrome or chromic oxides, resins having mixed
therein carbon black or coloring agents, and the like are examples
thereof.

[0119]As illustrated in FIGS. 2 and 3, the lens array 6, the
light-emitting element array 7, the spacer 84, the light shielding
members 81 and 82, and the diaphragm member 83 are collectively
accommodated in the casing 9. The casing 9 has a frame member (casing
body) 91, a lid member (bottom lid) 92, and a plurality of clamp members
93 that fixedly secure the frame member 91 to the lid member 92 (see FIG.
3).

[0120]The frame member 91 has a generally long shape, as illustrated in
FIGS. 2, 5, and 6.

[0121]In addition, the frame member 91 has a frame shape, and an inner
cavity portion 911 that is open to the upper and lower sides of the frame
member 91 is formed in the frame member 91 as illustrated in FIG. 3. The
width of the inner cavity portion 911 gradually decreases upwardly from
the lower side of FIG. 3.

[0122]The lens array 6', the spacer 84, the lens array 6, the light
shielding member 82, the diaphragm member 83, the light shielding member
81, and the light-emitting element array 7 are inserted in the inner
cavity portion 911, and they are fixed by adhesive, for example. In this
way, the lens array 6', the spacer 84, the lens array 6, the light
shielding member 82, the diaphragm member 83, the light shielding member
81, and the light-emitting element array 7 are collectively held on the
frame member 91, such that the positions in the main and sub-scanning
directions of the lens array 6', the spacer 84, the lens array 6, the
light shielding member 82, the diaphragm member 83, the light shielding
member 81, and the light-emitting element array 7 are determined.

[0123]An upper surface 722 of the supporting plate 72 of the
light-emitting element array 7 is in contact (abutting contact) with a
stepped portion 915, which is formed on a wall surface of the inner
cavity portion 911, and the lower surface of the second light shielding
member 81. The lid member 92 is inserted into the inner cavity portion
911 from the lower side.

[0124]The lid member 92 is formed of a lengthy member having a recess
portion 922 in which the accommodation portion 73 is inserted at an upper
side thereof. The edge portions of the support portion 72 of the
light-emitting element array 7 are pinched between the upper end surface
of the lid member 92 and the boundary portion 915 of the frame member 91.

[0125]Moreover, the lid member 92 is pressed upward by each of the clamp
members 93. In this way, the lid member 92 is fixed to the frame member
91. In addition, by the pressed lid member 92, the positional
relationships among the light-emitting element array 7, the light
shielding members 81 and 82, the diaphragm member 83, and the lens array
6 in the main-scanning direction, the sub-scanning direction, and the up
and down direction of FIG. 3 are fixed.

[0126]The clamp members 93 are preferably arranged in plural numbers at
equal intervals in the main-scanning direction. Accordingly, the frame
member 91 and the lid member 92 are pinched uniformly in the
main-scanning direction.

[0127]The clamp member 93 has a generally U shape in the cross section of
FIG. 3 and is formed by folding a metallic plate. Both ends of the clamp
member 93 are bent inward to form claws portions 931. The claw portions
931 are engaged with shoulder portions 916 of the frame member 91.

[0128]A curved portion 932 that is curved upward in an arch shape is
formed in the middle portion of the clamp member 93. The apex of the
curved portion 932 is in pressure-contact with the lower surface of the
lid member 92 in a state where the claw portions 931 are engaged with the
shoulder portion 916. In this way, the curved portion 932 urges the lid
member 92 upwardly in a state where the curved portion 932 is elastically
deformed.

[0129]When the clamp members 93 that pinch the frame member 91 and the lid
member 92 are detached, the lid member 92 can be detached from the frame
member 91. Maintenance, such as replacement and repair, can then be
performed for the light-emitting element array 7.

[0130]The constituent materials of the frame member 91 and the lid member
92 are not particularly limited, and the same constituent materials as
the supporting plate 72 may be used, for example. The constituent
materials of the clamp member 93 are not particularly limited, and
aluminum or stainless steel may be used, for example. In addition, the
clamp member 93 may also be formed of a hard resin material.

[0131]Although not illustrated in the drawings, the frame member 91 has
spacers that are provided at both ends in the longitudinal direction
thereof so as to protrude upward. The spacers are configured to regulate
the distance between the light receiving surface 111 and the lens array
6.

Optical System

[0132]The optical system 60 of the line head 13 is now described in detail
with reference to FIGS. 5-11.

[0133]As described above, in the line head 13, a pair of lenses 64 and 64'
corresponding to the light-emitting element group 71 are arranged in the
optical axis direction (namely, the lenses 64 and 64' are arranged in the
symmetrical axis direction of the lens 64). As illustrated in FIG. 5,
this pair of lenses 64 and 64' constitutes the optical system 60 that
images the light L from the light-emitting elements 74 belonging to the
corresponding light-emitting element group 71.

[0134]In the following description, a cross section (first cross section)
that contains the optical axis 601 and is in parallel to the
main-scanning direction is referred to as a "main-cross section", and a
cross section (second cross section) that contains the optical axis 601
and is perpendicular to the main-cross section is referred to as a
"sub-cross section". FIG. 5 illustrates a view of the optical system 60
taken along the main-cross section. In the following description, if
necessary, the optical system 60 formed by a pair of lenses 64a and 64a'
is referred to as an "optical system 60a", the optical system 60 formed
by a pair of lenses 64b and 64b' is referred to as an "optical system
60b", and the optical system 60 formed by a pair of lenses 64c and 64c'
is referred to as an "optical system 60c". The light-emitting elements
74a, 74b, 74c, and 74d are disposed at different positions in the
main-scanning direction (first direction), and an arbitrary two of
light-emitting elements 74a, 74b, 74c, and 74d constitute first or second
light-emitting elements.

[0135]The optical system 60 images the light L having passed through the
through-holes (aperture diaphragm) 831 of the diaphragm member 83 in the
vicinity of the light receiving surface 111 of the photoconductor 11.

[0136]In this embodiment, the optical system 60 is plane-symmetrical
(mirror-symmetrical) with respect to a symmetry plane (first symmetry
plane) perpendicular in the main-scanning direction (first direction),
and the optical system 60 is plane-symmetrical (mirror-symmetrical) with
respect to a symmetry plane (second symmetry plane) perpendicular in the
sub-scanning direction (second direction). An intersecting line of these
two symmetry planes is referred to as the axis of symmetry.

[0137]When the optical system 60 is rotationally symmetrical, the axis of
symmetry is identical to the optical axis. However, when the optical
system 60 is not rotationally symmetrical, strictly speaking, the optical
axis of the optical system 60 is not defined. In the following
description, the above-described axis of symmetry is described as the
optical axis for convenience sake.

[0138]As illustrated in FIG. 5, in the optical system 60, when light is
incident from the side of the light-emitting element 74, the light is
imaged at different positions (imaging points FP0, FP1, and FP2)
depending on the portion of the optical system 60 through which the light
passes. In FIG. 5, the rays of light are illustrated, assuming that light
emitted from an intersecting point (hereinafter referred to as a "virtual
light source") of the lower surface 721 of the support portion 72 and the
optical axis 601 is incident to the optical system 60.

[0139]The imaging point FP0 is a position (paraxial imaging point) at
which, when the light emitted from the virtual light source is incident
in the vicinity of the optical axis 601 of the optical system 60, the ray
of light (the emitted light) intersects the optical axis 601. The imaging
point FP1 is the position closest to the optical system 60 among the
positions at which, when the light emitted from the virtual light source
is incident to the optical system 60 via the diaphragm member 83, the ray
of light (the emitted light) intersects the optical axis 601. The imaging
point FP2 is the position farthest from the optical system 60 among the
positions at which, when the light emitted from the virtual light source
is incident to the optical system 60 via the diaphragm member 83, the ray
of light (the emitted light) intersects the optical axis 601.

[0140]That is, the optical system 60 has a longitudinal aberration on the
side of the optical system 60 and the opposite side with respect to the
imaging point FP0. That is, the optical system 60 has a longitudinal
aberration of which the signs are reversed with respect to an image
surface I. Here, the distance between the imaging points FP1 and FP2
corresponds to the difference between the maximum and minimum values of
the longitudinal aberration.

[0141]In the optical system 60, the spot size of the light L from the
light-emitting element 74 can be made to be small and substantially
constant for the ray of light imaged at an imaging point located between
the imaging point FP1 and the imaging point FP2 that are respectively
located closet and furthest from the optical system 60, among the
plurality of imaging points FP0, FP1, and FP2.

[0142]In particular, when the difference between the maximum and minimum
values of the longitudinal aberration in the light passing region of the
optical system 60 is defined as G, the distance Pel between the
light-emitting elements 74 in the main-scanning direction (first
direction) is defined as Pel, and the optical magnification of the
optical system 60 is defined as β, the following relation is
satisfied.

G>|β|Pel (1)

[0143]By making sure that the relation of (1) is satisfied, when the light
emitted from the light-emitting element 74 is imaged by the optical
system 60, the spot size of the light is made substantially constant over
a relatively wide range in the optical axis direction in the vicinity of
the image surface I (that is, the focal depth is increased). Therefore,
even when the positional relationship in the optical axis direction
between the image surface I and the light receiving surface 111, which is
the light irradiation surface, is changed or offset, variation of the
spot size on the light receiving surface ill is prevented. As a result, a
high-accuracy exposure process is realized.

[0144]As illustrated in FIG. 6, for example, when the separation distance
between the center of the light-emitting element 74d and the optical axis
601 is defined as Xo, and the separation distance between an imaging
position IFP of the light L4 from the light-emitting element 74d and the
optical axis 601 is defined as Xi, the optical magnification β
of the optical system 60 satisfies the relation of

|β|=|Xi|/|Xo|.

[0145]Moreover, as illustrated in FIG. 7, as described above, the
light-emitting element group 71 includes the eight light-emitting
elements 74 that are arranged in a matrix of two rows by four columns,
and the distance Pel is the distance between the centers of two
adjacent light-emitting elements 74 as seen from the main-cross section.
Here, the centers of the light-emitting elements are the centers of
geometry of the light-emitting elements, and the distance Pel is the
distance in the main-scanning direction between the centers of geometry
of the light-emitting elements.

[0146]As described above, the light emitted from the two adjacent
light-emitting elements 74 as seen from the main-cross section forms two
adjacent pixels on the light receiving surface 111. Therefore,
|β|Pel corresponds to the pixel pitch on the light receiving
surface 111. For example, in the case of forming a 1200-dpi image, the
pixel pitch is 21.166 μm.

[0147]By making sure that the optical system 60 has the front-side focal
plane FP0 in the vicinity of the optical axis 601 so as to be located
between the imaging points FP1 and FP2, the distance between the imaging
points FP1 and FP2 (namely, the difference G between the maximum and
minimum values of the longitudinal aberration) can be increased while
satisfying other optical characteristics needed by the optical system 60.
As a result, when the light emitted from the light-emitting elements 74
is imaged by the optical system 60, the spot size is made substantially
constant over a relatively wide range in the optical axis direction in
the vicinity of the image surface.

[0148]Therefore, even when the positional relationship in the optical axis
direction (third direction) between the image surface I and the light
receiving surface 111, which is the light irradiation surface, is changed
or offset, variation of the spot size on the light receiving surface 111
is prevented. As a result, concentration unevenness of formed latent
images is prevented.

[0149]Furthermore, the optical system 60 is preferably configured such
that the distance in the optical axis direction between the imaging
points FP2 and FP1, which are respectively located furthest and closet
from the optical system 60 (namely, the difference G between the maximum
and minimum values of the longitudinal aberration on the main-cross
section of the optical system 60), is larger that the spot size of the
light emitted from the light-emitting element 74 (namely, the spot size
on the image surface I). By doing so, variation of the spot size on the
light receiving surface 111 is effectively prevented.

[0150]The optical system 60 having such characteristics can be realized by
a multifocal lens having different focal points.

[0151]In this embodiment, the lenses 64 are configured as multifocal
lenses having a plurality of focal points, and the lenses 64' are
configured as single focus lenses having a single focal point so that the
optical system 60 is configured to have the plurality of above-described
imaging points FP1, FP1, and FP2.

[0152]As illustrated in FIGS. 8A and 8B, the lens 64 is formed on the
support portion 65 that is formed of a glass material, for example. The
lens 64 has a lens surface 62 on an opposite side to the support portion
65.

[0153]As illustrated in FIG. 9, the lens surface 62 of the lens 64 is
formed so that the lens 64 has a plurality of focal points fp0, fp1, and
fp2 that are located at different positions in the optical axis
direction.

[0154]The focal point fp0 is a position (paraxial focal point) at which,
when light parallel to the optical axis 601 (namely, light from infinite
distance) is incident in the vicinity of the optical axis 601 of the lens
64, the ray of light (the emitted light) intersects the optical axis 601.
The focal point fp1 is the position closest to the lens 64 among the
positions at which, when light parallel to the optical axis 601 is
incident to the lens 64 via the diaphragm member 83, the ray of light
(the emitted light) intersects the optical axis 601. The focal point fp2
is the position farthest from the lens 64 among the positions at which,
when light parallel to the optical axis 601 is incident to the lens 64
via the diaphragm member 83, the ray of light (the emitted light)
intersects the optical axis 601.

[0155]That is, the lens 64 has a longitudinal aberration on the side of
the lens 64 and the opposite side with respect to the focal point fp0.
The difference between the maximum and minimum values of the longitudinal
aberration corresponds to the distance g between the focal points fp1 and
fp2.

[0156]More specifically, as illustrated in FIGS. 8A and 8B, the lens
surface 62 of the lens 64 has a first circular region 62a that is defined
at a central portion thereof (so as to include the intersection with the
axis of symmetry of the lens 64) and a second ring-shaped region 62b that
is defined around the first region 62a. In FIG. 8B, a region, through
which the light having passed through the diaphragm member (aperture
diaphragm) 83 passes, is indicated by broken lines.

[0157]The surface shapes of the first region 62a and the second region 69b
are defined by different definition formulas. As the definition formula,
a definition formula (rotationally symmetrical aspheric surface)
expressed by Formula 1 below can be used, for example (see Examples below
for more details). In this way, the lens 64 having the above-described
characteristics is realized relatively easily and reliably.

[0163]The respective coefficients A to C and Δ of the definition
formula are appropriately set in accordance with the focal distance of
the optical system 60, the shape of the lens surface 62' of the lens 64',
and the like so that the optical system 60 has a plurality of
above-described imaging points.

[0164]When at least one of the coefficients A to C and Δ of the
definition formula is changed, the first region 62a and the second region
62b will be expressed by different definition formulas.

[0165]The optical axis in the definition formula refers to the axis of
symmetry of a rotationally symmetrical lens.

[0166]The size of the first region 62a is larger than the size of the
second region 62b. Thereby, the size of the first region 62a within the
light passing region a can be made to be substantially the same as the
size of the second region 62b within the light passing region a. As a
result, even when the positions in the optical axis direction of the
image surface and the light receiving surface 111 are changed, light
amount unevenness (concentration unevenness) of the spots formed on the
light receiving surface 111 is suppressed.

[0167]Moreover, as described above, the optical system 60 has a plurality
of lenses 64 and 64' that are arranged in the optical axis direction
thereof. An aperture diaphragm-side lens surface of the lens 64 disposed
closest to the aperture diaphragm 83 is configured as the above-described
lens surface 62 having the first region 62a and the second region 62b.
Therefore, light emitted from light-emitting elements 74 that are located
at different distances from the optical axis 601 (that is, even when the
light-emitting elements 74 provide different angles of view) is prevented
from being offset in the light passing region on the lens surface 62.
Accordingly, the above-described advantage of increasing the focal depth
of the optical system 60 is obtained by the other light-emitting elements
74.

[0168]Moreover, since the lens surface 62 including the first region 62a
and the second region 62b is provided on the side of the lens 64 close to
the light-emitting elements 74, characteristic variation due to an angle
of view is suppressed.

[0169]Similar to the lens 64, the lens 64' is formed on a support portion
65' that is formed of a glass material, for example. The lens 64' has a
lens surface 62' on an opposite side to the support portion 65'.

[0170]The lens surface 62' of the lens 64' may be a spherical or an
aspheric surface, and a surface shape thereof can be defined by one
definition formula. A definition formula (xy polynomial surface)
expressed by Formula 2 below can be used, for example (see Examples below
for more details).

[0177]The respective coefficients A to I of the definition formula are
appropriately set in accordance with the focal distance of the optical
system 60, the shape of the lens surface 62 of the lens 64, and the like
so that the optical system 60 has a plurality of above-described imaging
points.

[0178]The optical system 60 is configured such that the second region 62b
is provided closer to the outer periphery of the lens surface 62 than the
first region 62a and is in contact with the first region 62a. However,
the following relation is preferably satisfied when, among the light
emitted from the light-emitting elements 74, the minimum value of a
longitudinal aberration of light passing through the first region 62a
(namely, the longitudinal aberration of light having passed through the
vicinity of a boundary thereof) is defined as Δ1, and the
maximum value of a longitudinal aberration of light having passed through
the second region 62b (namely, the longitudinal aberration of light
having passed through the vicinity of a boundary thereof) is defined as
Δ2.

Δ2-Δ1=G (2)

[0179]By doing so, the difference G between the maximum and minimum values
of the longitudinal aberration of the optical system 60 can be
effectively increased.

[0180]In particular, in this embodiment, the longitudinal aberration
Δ1 is negative, and the longitudinal aberration Δ2
is positive. Therefore, the difference between the longitudinal
aberration Δ1 and the longitudinal aberration Δ2
can be effectively increased. Furthermore, the difference G between the
maximum and minimum values of the longitudinal aberration can be
effectively increased.

[0181]As illustrated in FIGS. 8A and 8B, when the lens surface 62 is
configured by two regions (the first region 62a and the second region
62b) having different definition formulas, the first region 62a includes
the center of the lens surface 62. Alternatively, as illustrated in FIGS.
11A and 11B, a lens 64A may be used that has a lens surface 62A
configured by three regions 62c, 62d, and 62e having different definition
formulas, in lieu of the lens surface 62.

[0182]The region (third region) 62c includes the intersection with the
axis of symmetry of the lens 64A. The region (first region) 62d is closer
to the outer periphery of the lens surface 62A than the region 62c and is
in contact with the region 62c. The region 62e is closer to the outer
periphery of the lens surface 62A than the region 62d and is in contact
with the region 62d.

[0183]As described above, the lens surface 62A illustrated in FIGS. 11A
and 11B includes the region (third region) 62c that is provided closer to
the inner periphery of the lens surface 62A than the region (first
region) 62d so as to include the center of the lens surface 62A and be in
contact with the region 62d and that has a surface shape that is defined
by a definition formula different from that of the region 62d. In this
case, when, among the light emitted from the light-emitting elements 74,
a longitudinal aberration of light having passed through the region 62d
in the vicinity of its boundary with the region 62e is defined as
Δ1, and a longitudinal aberration of light having passed
through the region 62e in the vicinity of its boundary with the region
62d is defined as Δ2, by making sure that the above-described
relation (2) is satisfied, the difference G between the maximum and
minimum values of the longitudinal aberration of the optical system 60
can be effectively increased.

[0184]In the optical system 60 having the above-described configuration,
the light L (L1, L2, L3, and L4) emitted from the four light-emitting
elements 74 (74a, 74b, 74c, and 74d), which are linearly arranged in the
main-scanning direction as illustrated in FIGS. 5 and 6, are sequentially
permitted to pass through the lens 64 and the lens 64' after passing
through the diaphragm member 83. In this way, the respective rays of
light L1, L2, L3, and L4 are imaged (focused) in the vicinity of the
light receiving surface 111 of the photoconductor 11 as illustrated in
FIG. 10.

[0185]At that time, by the above-described function of the optical system
60 having a plurality of imaging points, the light L1 is imaged at a
plurality of imaging positions IFP10, IFP11, and IFP12 that are located
at different positions in its traveling direction (third direction).

[0186]The imaging position IFP10 is a position (paraxial imaging point) at
which, when the light L1 emitted from the light-emitting element 74a is
incident to the lens 64 via the diaphragm member 83, the ray of light
passing through the vicinity of the optical axis 601 is imaged (focused).
The imaging position IFP11 is the position closest to the optical system
60 among the positions at which, when the light L1 emitted from the
light-emitting element 74a is incident to the lens 64 via the diaphragm
member 83, the ray of light passing through the first region 62a of the
lens 64 is imaged (focused). The imaging position IFP12 is the position
furthest from the optical system 60 among the positions at which, when
the light L1 emitted from the light-emitting element 74a is incident to
the lens 64 via the diaphragm member 83, the ray of light passing through
the second region 62b of the lens 64 is imaged (focused).

[0187]Similarly, the light L2 is imaged at a plurality of imaging
positions IFP20, IFP21, and IFP22 that are located at different positions
in its traveling direction (third direction). Moreover, the light L3 is
imaged at a plurality of imaging positions IFP30, IFP31, and IFP32 that
are located at different positions in its traveling direction (third
direction). Furthermore, the light L4 is imaged at a plurality of imaging
positions IFP40, IFP41, and IFP42 that are located at different positions
in its traveling direction (third direction).

[0188]The respective rays of light L1, L2, L3, and L4 imaged by the
optical system 60 will have their spot sizes that are substantially
constant over a relatively wide range (distance G1) in the optical axis
direction in the vicinity of the image surface.

[0189]The optical system 60 is configured so that the imaging positions
IFP10, IFP20, IFP30, and IFP40 are located in the vicinity of the light
receiving surface 111.

[0190]Therefore, even when the positional relationship in the optical axis
direction (third direction) between the image surface I and the light
receiving surface 111, which is the light irradiation surface, is changed
or offset, the light receiving surface 111 is positioned between the
imaging positions IFP11 and IFP12, between the imaging positions IFP21
and IFP22, between the imaging positions IFP31 and IFP32, and between the
imaging positions IFP41 and IFP42.

[0191]In this way, the line head 13 prevents a variation of the spot size
on the light receiving surface 111. As a result, concentration unevenness
of formed latent images is prevented.

[0192]FIG. 10 illustrates a case where the optical system 60 has an
image-surface curvature. Specifically, the imaging position IFP10 of the
light L1, the imaging position IFP20 of the light L2, the imaging
position IFP30 of the light L3, and the imaging position IFP40 of the
light L4 are located on a curved image surface I. Therefore, the imaging
positions IFP10 and IFP40 and the imaging positions IFP20 and IFP30 are
offset from each other in the optical axis direction.

[0193]More specifically, as illustrated in FIGS. 5 and 6,, among the four
light-emitting elements 74 (74a, 74b, 74c, and 74d) arranged linearly in
the main-scanning direction, two light-emitting elements 74b and 74c are
located at positions close to the optical axis 601 of the optical system
60, and the other two light-emitting elements 74a and 74d are located at
positions distant from the optical axis 601. The light-emitting elements
74a and 74d and the light-emitting elements 74b and 74c have different
angles of view. As a result, there is a case where the imaging positions
IFP10 and IFP40 and the imaging positions IFP20 and IFP30 are sometimes
offset in the optical axis direction (third direction) due to the
image-surface curvature of the optical system 60.

[0194]In such a case, the above-described distance between the imaging
point FP1 and the imaging point FP2 (namely, the difference G between the
maximum and minimum values of the longitudinal aberration) is larger than
the maximum value G1 of the offset amount. Therefore, even when the image
surface I of the optical system 60 and the light receiving surface 111
are slightly offset in the optical axis direction, the difference on the
light receiving surface 111 between the spot size of the light from the
light-emitting element 74 that is positioned closer to the optical axis
601 and the spot size of the light from the light-emitting element 74
that is positioned distant from the optical axis 601 can be decreased.

[0195]Furthermore, even when the positional relationship between the image
surface I of the optical system 60 and the light receiving surface 111 is
offset or varied due to errors in mounting the line head 13 onto the body
of the image forming apparatus 1, eccentricity of the photosensitive drum
11, or the like, variation of the spot size on the light receiving
surface 111, of the light from the light-emitting elements 74, can be
prevented.

[0196]Although a line head and an image forming apparatus according to the
embodiments of the invention have been described, the invention is not
limited thereto. Each of the components provided in the line head and the
image forming apparatus can be replaced with a component having an
arbitrary configuration that realizes the same function. In addition, an
arbitrary structure may be added.

[0197]Furthermore, in the lens arrays, a plurality of lenses is not
limited to being arranged in a matrix of two rows by n columns. For
example, a plurality of lenses in each of the lens arrays may be arranged
in a matrix of three rows by n columns, four rows by n columns, and the
like.

[0198]Moreover, one optical system may be configured by a plurality of
lenses, and may be configured to have one or three or more lens surfaces.

[0199]Furthermore, in the above-described embodiment, although the
light-emitting elements are described as being arranged in a matrix of
one row by n columns for the convenience of explanation, the arrangement
is not limited to this, and the light-emitting elements may be arranged
in a matrix of two rows by n columns, three rows by n columns, and the
like.

EXAMPLES

[0200]Specific examples of the invention are now described.

Example

[0201]A line head having the optical system as illustrated in FIG. 12 was
designed, and the characteristics thereof were evaluated by simulation.
FIG. 12 is a cross-sectional view taken along the main-cross section,
illustrating the optical system included in the line head according to an
Example of the invention.

[0202]The line head of the present example had the same configuration as
the line head illustrated in FIGS. 3 and 5, except that three
light-emitting elements 74 were arranged in the main-scanning direction.

[0203]Here, in the main-cross section, the three light-emitting elements
74 arranged in the main-scanning direction were arranged symmetrically to
the optical axis.

[0204]A glass material was used as the constituent materials of the
support portions 65 and 65', and a resin material was used as the
constituent materials of the lenses 64 and 64'.

[0205]The surface configuration of the optical system of the line head is
shown in Table 1.

[0206]As illustrated in FIG. 12, in Table 1, a surface S1 is a boundary
surface (light source plane) of the light-emitting element 74 and the
support portion 72, a surface S2 is a surface (emission surface of a
glass substrate) of the support portion 72 opposite to the light-emitting
element 74, a surface S3 is a surface (aperture diaphragm) of the
diaphragm member 83 close to the light-emitting element 74, a surface S4
is the lens surface 62 (incidence surface of a resin portion) of the lens
64, a surface S5 is a boundary surface (resin-glass boundary surface) of
the lens 64 and the support portion 65, a surface S6 is a surface
(emission surface of the glass substrate) of the support portion 65
opposite to the lens 64, a surface S7 is the lens surface 62' (incidence
surface of the resin portion) of the lens 64', a surface S8 is a boundary
portion (resin-glass boundary surface) of the lens 64' and the support
portion 65', a surface S9 is a surface (emission surface of the glass
substrate) of the support portion 65' opposite to the lens 64', and a
surface S10 is the light receiving surface 111 (image surface).

[0207]Moreover, a surface spacing d1 is a spacing between the surface S1
and the surface S2, a surface spacing d2 is a spacing between the surface
S2 and the surface S3, a surface spacing d3 is a spacing between the
surface S3 and the surface S4, a surface spacing d4 is a spacing between
the surface S4 and the surface S5, a surface spacing d5 is a spacing
between the surface S5 and the surface S6, a surface spacing d6 is a
spacing between the surface S6 and the surface S7, a surface spacing d7
is a spacing between the surface S7 and the surface S8, a surface spacing
d8 is a spacing between the surface S8 and the surface S9, and a surface
spacing d9 is a spacing between the surface S9 and the surface S10.

[0208]Furthermore, a refractive index at reference wavelength is the
refractive indexes on the respective surfaces with light having the
reference wavelength.

[0209]The wavelength (reference wavelength) of the light emitted from the
light-emitting element 74 was 690 nm, the object-side numerical aperture
was 0.153, the total width of the object-side pixel group in the
main-scanning direction was 1.176 mm, and the total width of the
object-side pixel group in the sub-scanning direction was 0.127 mm.

[0210]The distance (pitch) Pel between adjacent light-emitting
elements in the main-scanning direction was 0.042 mm, and the optical
magnification β of the optical system was -0.5039.

[0211]The lens surface 62 of the lens 64 was configured such that a range
of regions within a radius of 0 to 0.604 mm around the optical axis was
defined as the first region, and a range of regions outside the radius
0.604 mm around the optical axis was defined as the second region. The
surface shapes of the respective regions were defined using the
coefficients shown below in the definition formula given by Formula 1.

Coefficients of Definition Formula of the First Region of the Lens Surface
62

[0236]The optical system obtained in the above-described manner had a
longitudinal aberration as shown in FIG. 13. In FIG. 13, the horizontal
axis is defined such that, when the 0 (reference) point of the horizontal
axis corresponds to a longitudinal aberration in the vicinity of the
optical axis, the left side is the light source side and the right side
is the image side. The vertical axis represents the separation distance
of the ray of light having passed through the diaphragm member 83 from
the optical axis when the center of the diaphragm member (aperture
diaphragm) 83 is at 0, and the radius of the through-hole (opening) of
the diaphragm member 83 is set to 1.

Comparative Example

[0237]A line head was designed similar to the above-described example,
except that the surface shape of the lens surface 62 of the lens 64 was
made identical to the surface shape of the lens surface 62' of the lens
64', and the characteristics thereof were evaluated by simulation.

Evaluation

[0238]FIGS. 14A and 14B respectively illustrate changes in the spot sizes
at various positions in the optical axis direction of the optical systems
of the example and the comparative example. FIG. 14A illustrates the
example of the invention, and FIG. 14B illustrates the comparative
example.

[0239]As is evident from FIGS. 14A and 14B, the line head (optical system)
of the example according to the invention was better able to suppress a
change in the spot size in the vicinity of the minimum spot size than the
line head of the comparative example.

[0240]Moreover, when the line head of the example was mounted on the image
forming apparatus as shown in FIG. 1, high-quality images in which
concentration unevenness was suppressed were obtained.